Currently much effort is being expended in developing apparatus for transmitting video signals, for high definition television, (HDTV), telephonic applications, interactive video applications etc. One very attractive approach is known as motion compensated predictive encoding. The International Organization for Standardization is currently developing a standard specifying the coded representation of video for digital storage media, based on a variant of this type of encoding. This standard supports a continuous data transfer rate of 1.5 Mbits/sec., and is described in the document ISO-IEC JTC1/SC2/WG11; CODING OF MOVING PICTURES AND ASSOCIATED AUDIO; MPEG90/176 Rev.2, Dec. 18, 1990. This format has become known as MPEG. According to this format sequences of frames are divided into groups, and respective frames within each group are encoded according to one of a plurality of coding modes. Typically the coding modes include intraframe coding, (I frames) and two types of interframe predictive coding (P and B frames).
The Advanced Television Research Consortium (ATRC) in the United States has developed an HDTV system based upon the MPEG format, for transmission of high definition television (HDTV) signals in digital form. This HDTV System is described in U.S. Pat. No. 5,122,875. In the HDTV system developed by the ATRC, video signal compressed according to an MPEG like format is arranged in service type specific transport packets for transmission. These packets undergo a first level of error encoding, to generate a frame check sequence FCS, e.g., a cyclic redundancy check, and FCS error check codes are appended to the transport packets. Thereafter the transport packets, with the appended error check codes, undergo a forward error coding, FEC, such as a Reed-Solomon coding, and FEC error detection/correction codes are appended to the data.
At the receiver, transmitted information is detected and applied to an FEC decoder, which performs a limited error correction function on the transmitted data. The FEC corrects the majority of errors incurred during transmission, however because the error check overhead volume is limited by bandwidth constraints, some errors will pass the FEC decoder without detection/correction. The FEC decoded signal is then coupled to an FCS decoder. The FCS decoder is capable of detecting but not correcting data. If an error is detected in a transport packet, the entire transport packet is discarded.
The discarded transport packets may thereafter be replaced with synthesized compressed data. An example of apparatus for performing pre-decompression error concealment is described in copending application Ser. No. 07/789,245 filed Nov. 7, 1991 and entitled "Apparatus For Concealing Errors In A Digital Video Processing System." In this system the FCS decoder generates error tokens which indicate when data has been discarded. Responsive to the error tokens the system substitutes predetermined sequences of compressed data for discarded data. The data is then decompressed for display or storage purposes. Depending upon the type of data that is lost, replacement by synthesized compressed data may not produce satisfactory images.